The APOBEC3 (A3)-Vif interaction dictates whether HIV 'lives'or 'dies'. In a normal infection, HIV uses its auxiliary protein Vif to neutralize the cellular retroviral restriction factors A3G and A3F. Conversely, in the absence of Vif, these A3 proteins are able to potently inhibit HIV infectivity. The A3-Vif interaction has therefore become a prime target for the development of novel therapeutic interventions. However, a current impediment to therapeutic development is a vast knowledge gap owing to the fact that there are precious few structural, biophysical and biochemical studies on these A3 proteins or HIV Vif. To fill this gap, we have assembled a truly multidisciplinary Program Project that combines the strengths of five laboratories, each with complementary sets of expertise in molecular virology, NMR spectroscopy, X-ray crystallography, AFM force spectroscopy and biochemistry. Our projects are assembled to achieve the following broad, high-impact objectives: (i) elucidate the full-length A3G structure and gain a comprehensive understanding for how this protein oligomerizes during HIV restriction, (ii) define how A3G binds single-strand DNA and catalyzes cytidine to uridine deamination, and (iii) dissect the Vif interaction surfaces of A3G and A3F to fully understand critical similarities and differences. We will also use novel A3G inhibitors and small peptides as molecular probes to dissect stages of the DNA deamination mechanism. Each investigator will apply his/her specific expertise to each of these aims and our team will work toward building-up the 'big picture'for how A3F and A3G mediate HIV-1 restriction and how Vif counteracts these multifaceted and potent innate immune defenses. Overall, this Program Project will provide unprecedented atomic, biophysical, biochemical, and molecular information. We anticipate that this knowledge will help accelerate the development and implementation of novel HIV/AIDS therapeutics that work by leveraging the A3/Vif axis.